#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main() {
    int s[30] = { 1,2 };
    int input;
    scanf("%d", &input);
    for (int i = 2; i < input; i++) {
        s[i] = s[i - 1] + s[i - 2];
    }
    printf("%d", s[input - 1]);
    return 0;
}